For those races that consistently have less than 5 members, has it been thought to ever make those races NPC only? (not sure on a timeframe, but at least 1 year) ...then to have those members pick a new race?

Really, it's a sign that certain individuals need to be more selective in their race implementation. :p

But now that the races are here, there is little sense in removing them. The NPC-restricted races that we have are restricted for legitimate reasons, such as certain abilities that cannot be implemented in SWC, highly obscure/arguably non-canon origins, or reasons of balance. If the races had no members at all perhaps they might be removed at some point, but thus far that has not been the case (and I'm not sure how that would address inactive members of the race--whether they would be taken into account at all).

I only recall one situation where a playable race containing a member was made NPC-only (Imzig), and the player in question (Zeblork) was permitted to remain that race regardless and was not forced to change.

Why woud you that? What's the point? Why should Combine have less members of rare races and more members of more popular races? I think the more races the admins can implement, the better, even if the member count of any race goes to 0, someone might come and start actively RPing it.